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Motti Lanzkron
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 Posted: Wed Dec 29, 2004 9:43 pm Post subject: Partial specialization of non-type template arguments |
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I'm trying to specialize a template according to a non-type argument
which is dependant on another template argument. None of the compilers
I tried it on gave the results I expected but I can't find anything
against it in the standard.
Here's a contrived example of what I'm trying to do:
template <int N>
struct Int {
static const int value = N;
};
template <class T, int N>
struct Eq {
static const bool value = false;
};
template <class T>
struct Eq<T, T::value> {
static const bool value = true;
};
bool check = Eq<Int<2>, 2>::value;
Here's the message I get from Comeau online:
"ComeauTest.c", line 12: error: a partial specialization nontype
argument must be the name of a nontype parameter or a constant
struct Eq<T, T::value> {
^
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Hyman Rosen
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| Posted: Thu Dec 30, 2004 3:13 am Post subject: Re: Partial specialization of non-type template arguments |
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Motti Lanzkron wrote:
| Quote: | I can't find anything against it in the standard.
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You didn't look hard enough: 14.5.4/9
A partially specialized non-type argument expression
shall not involve a template parameter of the partial
specialization except when the argument expression is
a simple _identifier_.
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Manuel Petit
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| Posted: Thu Dec 30, 2004 9:54 am Post subject: Re: Partial specialization of non-type template arguments |
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Motti Lanzkron wrote:
| Quote: | I'm trying to specialize a template according to a non-type argument
which is dependant on another template argument. None of the compilers
I tried it on gave the results I expected but I can't find anything
against it in the standard.
Here's a contrived example of what I'm trying to do:
template <int N
struct Int {
static const int value = N;
};
template
struct Eq {
static const bool value = false;
};
template
struct Eq
static const bool value = true;
};
|
Why write that when you can simply write:
template <class T, int N>
struct Eq {
static bool const value = N==T::value;
};
manuel,
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Graeme Prentice
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| Posted: Thu Dec 30, 2004 8:15 pm Post subject: Re: Partial specialization of non-type template arguments |
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On 30 Dec 2004 04:54:44 -0500, Manuel Petit wrote:
| Quote: | Motti Lanzkron wrote:
I'm trying to specialize a template according to a non-type argument
which is dependant on another template argument. None of the compilers
I tried it on gave the results I expected but I can't find anything
against it in the standard.
Here's a contrived example of what I'm trying to do:
template <int N
struct Int {
static const int value = N;
};
template
struct Eq {
static const bool value = false;
};
template
struct Eq
static const bool value = true;
};
Why write that when you can simply write:
template <class T, int N
struct Eq {
static bool const value = N==T::value;
};
|
I wondered that to start with, then I realized the intention could be
that when the Eq template is used like this
Eq
the specialization is wanted if somevalue is the same value as the
sometype::value, otherwise the primary template. If that's the case,
this can be achieved by inheriting one of two base classes where the
base class is selected something like this
template <class T, int N>
struct Eq : public somebase<T,T::value==N> {
};
except that if T doesn't have a suitable value member the code fails.
There's a chance it's possible to deal with this by using SFINAE so just
for fun, I tried but GCC complains for the following code if the lines
in main involving s2,s3 are uncommented whereas Comeau and VC7.1 are ok
with all of it.
The C++ standard SFINAE rules don't appear to allow the case of
template <typename T1>
yes & f1( typename x1<T1::value>::type *);
when T1::value is not being used as a type but Comeau, VC7.1 and GCC all
give no error if T1 has no value member.
#include <iostream>
#include <ostream>
template <int n>
struct x1
{
typedef int type;
};
typedef char yes;
typedef char no[2];
template <typename T1>
yes & f1( typename x1<T1::value>::type *);
template <typename T1>
no &f1(...);
struct s1 {
static const int value = 16;
};
struct s2 {
int * value;
};
struct s3 {
void value();
};
struct s4 {};
template <typename T, int N, bool b>
struct helper1 {
// has no value member matching N
static const bool result = false;
};
template <typename T, int N,bool b>
struct helper2_has_value_integral_const_member : helper1<T,N,false> {
};
template <typename T, int N>
struct helper2_has_value_integral_const_member<T,N,true> {
// has value member equal to N
static const bool result = true;
};
template <typename T, int N>
struct helper1<T,N,true> :
public helper2_has_value_integral_const_member
<T,N,T::value == N> {};
template <class T, int N>
struct Eq : public helper1<T,N,sizeof(f1 {
};
int main()
{
std::cout << (sizeof(f1
// << (sizeof(f1
// << (sizeof(f1
<< (sizeof(f1
<< Eq
<< Eq
// << Eq
// << Eq
<< Eq
}
Graeme
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Motti Lanzkron
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| Posted: Thu Dec 30, 2004 8:37 pm Post subject: Re: Partial specialization of non-type template arguments |
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Hyman Rosen wrote:
| Quote: | Motti Lanzkron wrote:
I can't find anything against it in the standard.
You didn't look hard enough: 14.5.4/9
A partially specialized non-type argument expression
shall not involve a template parameter of the partial
specialization except when the argument expression is
a simple _identifier_.
|
Right you are, and when I tried even harder I found that this has been
discussed in the past (I hope this link works):
http://groups.google.co.uk/groups?hl=en&lr=&threadm=us5snlond4or58%40corp.supernews.com
I see Rani Sharoni suggested using Int2Type type parameter instead of
the int non-type parameter (in my contrived example this is obviously
ludicrous but it fits the real problem I had). So it seems clear that
there is no real technical difficulty that makes 14.5.4/9 necessary.
Why is it there then?
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Hyman Rosen
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| Posted: Fri Dec 31, 2004 1:19 pm Post subject: Re: Partial specialization of non-type template arguments |
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Motti Lanzkron wrote:
| Quote: | Why is it there then?
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Let's modify your code slightly:
template <int N> struct Int { static const int value = N; };
template <int N> struct Eq { static const bool value = false; };
template <class T> struct Eq<T::value> { static const bool value = true; };
bool check = Eq<2>::value;
See?
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Motti Lanzkron
Guest
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| Posted: Mon Jan 03, 2005 12:36 am Post subject: Re: Partial specialization of non-type template arguments |
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Hyman Rosen wrote:
| Quote: | Motti Lanzkron wrote:
Why is it there then?
Let's modify your code slightly:
template <int N> struct Int { static const int value = N; };
template <int N> struct Eq { static const bool value = false; };
template <class T> struct Eq<T::value> { static const bool value = true; };
bool check = Eq<2>::value;
See?
|
Nope sorry, I'm completely devoid of sight.
I don't understand what you're trying to show with your example.
In any case how is it different from:
template <class T> struct Type { typedef T type; };
template <class T> struct Eq { static const bool value = false; };
template <class T> struct Eq<typename T::type>
{ static const bool value = true; };
bool check = Eq<2>::value;
This isn't legal either but you don't say that during specialization a
template type parameter must be a simple identifier.
I don't understand why the restriction in case of non-type parameters
is necessary. It seems that in every case there is a type equivalent
that is legal which means that the compiler could save us from the
proverbial extra level of indirection.
Consider the following:
// class with static buffer
struct Stat {
static char buf;
};
char Stat::buf;
#ifdef WHAT_I_EXPECT_TO_WORK
template <class T, char* P>
struct Test
{
static const bool value = false;
};
template <class T>
struct Test_<T, &T::buf>
{
static const bool value = true;
};
#else // !WHAT_I_EXPECT_TO_WORK
// Wrap a pointer to char with a type
template <char* P> struct Wrap {};
// extra level of indirection
template <class T, class Y>
struct Test_
{
static const bool value = false;
};
template <class T>
struct Test_<T, Wrap<&T::buf> >
{
static const bool value = true;
};
template <class T, char* P>
struct Test : Test_<T, Helper {};
char test1[Test<Stat, &Stat::buf>::value]; // OK
char test2[Test<int, &Stat::buf>::value]; // Error zero sized array
#endif
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